1. Field of the Invention
The present invention relates generally to an array antenna system. In particular, the present invention relates to an apparatus and method for optimal beam forming for transmitting and receiving high-speed data.
2. Description of the Related Art
Reception quality of radio signals is affected by many natural phenomena. One of the natural phenomena is temporal dispersion caused by signals reflected on obstacles in different positions in a propagation path before the signals arrive at a receiver. With the introduction of digital coding in a wireless system, a temporally dispersion signal can be successfully restored using a Rake receiver or equalizer.
Another phenomenon called fast fading or Rayleigh fading is spatial dispersion caused by signals which are dispersed in a propagation path by an object located a short distance from a transmitter or a receiver. If signals received through different spaces, i.e., spatial signals, are combined in an inappropriate phase region, the sum of the received signals is very low in intensity, approaching zero. This becomes a cause of fading dips where received signals substantially disappear, and the fading dips frequently occur when a length corresponds to a wavelength.
A known method of removing fading is to provide an antenna diversity system to a receiver. The antenna diversity system includes two or more spatially separated reception antennas. Signals received by the respective antennas have low relation in fading, reducing the possibility that the two antennas will simultaneously generate the fading dips.
Another phenomenon that significantly affects radio transmission is interference. The interference is defined as an undesired signal received on a desired signal channel. In a cellular radio system, the interference is directly related to a requirement of communication capacity. Because radio spectrum is a limited resource, a radio frequency band given to a cellular operator should be efficiently used.
Due to the spread of cellular systems, research is being conducted on an array antenna structure connected to a beam former (BF) as a new scheme for increasing traffic capacity by removing an influence of the interference and fading. Each antenna element forms a set of antenna beams. A signal transmitted from a transmitter is received by each of the antenna beams, and spatial signals experiencing different spatial channels are maintained by individual angular information. The angular information is determined according to a phase difference between different signals. Direction estimation of a signal source is achieved by demodulating a received signal. A direction of a signal source is also called a “Direction of Arrival (DOA).”
Estimation of DOAs is used to select an antenna beam for signal transmission to a desired direction or steer an antenna beam in a direction where a desired signal is received. A beam former estimates steering vectors and DOAs for simultaneously detected multiple spatial signals, and determines beam-forming weight vectors from a set of the steering vectors. The beam-forming weight vectors are used for restoring signals. Algorithms used for beam forming include Multiple Signal Classification (MUSIC), Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT), Weighted Subspace Fitting (WSF), and Method of Direction Estimation (MODE).
An adaptive beam forming process depends upon having correct information on spatial channels. Therefore, adaptive beam forming can be acquired only after estimation of the spatial channels. This estimation should consider not only temporal dispersion of channels but also DOAs of radio waves received at a reception antenna.
For estimation of spatial channels, a reception side requires the arrangement of an array antenna having Ka antenna elements. Such an array antenna serves as a spatial low-pass filter having a finite spatial resolution. The term “spatial low-pass filtering” refers to an operation of dividing an incident wave (or impinging wave) of an array antenna into spatial signals that pass through different spatial regions. A receiver having the foregoing array antenna combines a finite number, Nb, of spatial signals, for beam forming. As described above, the best possible beam forming requires information on DOAs and a temporally dispersed channel's impulse response for the DOAs. A value of the Nb cannot be greater than a value of the Ka, and thus represents the number of resolvable spatial signals. The maximum value, max(Nb), of the Nb is fixed according to a structure of the array antenna.
FIG. 1 illustrates an example of a base station (or a Node B) with an array antenna, which communicates with a plurality of mobile stations (or user equipments). Referring to FIG. 1, a base station 115 has an array antenna 110 comprised of 4 antenna elements. The base station 115 has 5 users A, B, C, D and E located in its coverage. A receiver 100 selects signals from desired users from among the 5 users, by beam forming. Because the array antenna 110 of FIG. 1 has only 4 antenna elements, the receiver 100 restores signals from a maximum of 4 users, e.g., signals from users A, B, D and E as illustrated, by beam forming.
FIG. 2 illustrates spatial characteristics of beam forming for selecting a signal from a user A, by way of example. As illustrated, if a very high weight, or gain, is applied to a signal from a user A, a gain approximating zero is applied to signals from the other users.
In an antenna diversity system using an array antenna, resolvable beams are associated with DOAs of max(Nb) maximum incident waves. Actually, the total number of incident waves is much greater than max(Nb), and is subject to change according to a mobile environment. In order to achieve beam forming, a receiver should acquire information on DOAs, and the acquisition of DOA information can be achieved through DOA estimation. However, estimated DOAs are not regularly spaced apart from each other. Therefore, in a digital receiver, conventional beam forming includes irregular spatial samplings. A final goal of beam forming is to separate an incident wave so as to fully use spatial diversity in order to suppress fading. However, its latent faculty is limited by the structure of an array antenna having a finite spatial resolution.
In the conventional beam-forming methods, differentiation between spatially selective transmission channels for radio mobile communication includes three separate steps a first step of estimating spatial channels, a second step of estimating DOAs based on the estimated spatial channels, and a third step of estimating a spatial and temporal channel impulse characteristic for a beam forming algorithm using the estimated spatial channels and the estimated DOAs. This 3-step method has a heavy implementation load and causes considerable signal processing cost in operation and a lack of robustness due to estimation errors.